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Work and Energy
WORK
Work measures the effects of a force acting
over a distance.
W = F*d
The units are [N]*[m] = [Joules] = [J]
F* d = W
WORK
W = F*d
WORK
Q: You can lift a maximum of 1000 Newtons.
If you try to lift a car and it doesn’t
move…How much work have you done?
F= 1000 Newtons = 1000 N
d= 0 meters = 0 m
A: NONE!!! W = 0 J because the distance
was 0 m and W = F * (0 m) = 0 J
Work Practice Problems Pg. 285
1.) F = 5200N
d = 25m
W = F*d
W=?J
= (5200N)*(25m) = 130,000J
2.) F = 1N
d = 1m
W=?J
W = F*d
= (1N)*(1m) = 1J
Work Practice Problems Pg. 285
3.) F = 125N
d = 14.0m
W = F*d
W=?J
= (125N)*(14.0m) = 1750J
4.) F = 165N
d = .800m
W = F*d
W = ? J = (165N)*(.800m) = 132J*30 = 3960J
Work Practice Problems Pg. 285
5.) d = .5m
F = m*g (g = 9.8m/s2)
m = 1200kg
F=?N
= (1200kg)*(9.8m/s2) =
12,000N
F = 12,000N
W = F*d
d = .5m
W = ?J
= (12,000N)*(.5m) = 5880J
ENERGY
When you are doing work you are
transferring energy.
There are several types of energy:
Mechanical, electrical, chemical, thermal
(heat), Potential and Kinetic.
Warm-Up
Ms. S. applies 40N while tightening a bolt on
her car tire. The bolt moves .23m. How
much work was done on the bolt?
What type of energy is stored in molecules
when they bond?
What type of energy is in an apple hanging
from a tree?
POTENTIAL energy (PE)
Potential Energy (PE) is the stored energy of an
object resulting from the position of the object in
a system.
PE is measured by multiplying the mass of the
object times the gravity and then times the
height.
PE = m*g*h
(g = 9.8m/s2)
Units are Joules!!! (J)
POTENTIAL energy (PE)
Examples of objects with high PE:
A stretched rubber band
Any hanging object
Any compressed object (like a spring)
Any object that is high up
Solving Energy Problems
Plug PE = m*g*h into a Power Triangle
Write the equations when you solve for m and h.
Solving Energy Problems
m = PE
g*h
h = PE
g*m
Answers to PE calculations
1.) PE = 6856.1J
5.) m = .35kg
2.) m = 4.7kg
6.) h = 24.7m
3.) h = 19.2m
4.) PE = 7763.9J
KINETIC energy (KE)
Kinetic Energy (KE) is the energy of any moving
object.
KE depends on the mass and the velocity (speed)
of an object. Only ½ of the mass is used, but
velocity is squared… so KE depends mostly on
the velocity of the object.
KE = ½m * v2
Solving Energy Problems
KE = ½ m * v2
***Remember PEMDAS (order of
operations)
1ST: Square the v
2nd: Take half the mass
3rd: Multiply 1/2m * v2
Warm-Up
1. A 3.4 kg defensive lineman moving at 2.1 m/s2
sacks the quarterback, who is standing still.
What is the net force of the collision?
2. Give an example of a specific object that has
high PE.
3. Give an example of a specific object that has
high KE.
Units Review
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Force = Newtons (N)
Speed or velocity = distance/time (d/t = m/s)
Acceleration = distance/time2 (d/t2 = m/s2)
Energy (Work, KE, PE, TE) = Joules (J)
Mass = kilograms (kg)
Distance or height = meters (m)
Time = seconds (s)
Power = Watts (W)
Answers to KE calculations
7.) KE = 18,021.61J
11.) KE = 1701.51J
8.) KE = 1292.93J
12.) KE = 2795.88J
9.) KE = 5006.07J
10.) KE = 38,423.15J
ENERGY TRANSFER
There are three ways that energy can be transferred:
Conduction – The transfer of energy through matter by
direct contact of particles.
Radiation – The transfer of energy in the form of a wave.
Convection – The transfer of energy by the movement of
gases or liquids. (only fluids!!!)
EXAMPLES of ENERGY
TRANSFER
Conduction: a pot on a stove, a metal
spoon in a bowl of soup…the handle gets
hot too.
Radiation: how the Earth gets energy from
the sun, a microwave
Convection: how refrigerators stay cold…
LAW OF CONSERVATION of
ENERGY
Energy cannot be created or destroyed; it can only
be transformed from one type to another or
transferred. (The TOTAL ENERGY (TE) stays
the same!!!) TE = PE + KE + heat
So when an object falls… how is energy
transformed?
It goes from PE to KE…
LAW OF CONSERVATION of
ENERGY
Let’s look at some systems where energy
transformation is taking place…(i.e. changing
from one type of energy to another type of
energy.)
A Bungee jumper
A Pendulum
A Hydroelectric Power Plant
HYDROELECTRIC POWER
PLANT
LAW OF CONSERVATION of
ENERGY
The TOTAL ENERGY of a system can be
measured by adding PE + KE + Heat.
TE = KE + PE + heat
Q: Where does the heat come from???
A: Friction!!! (Rub your hands together and see
what happens!!!)
LAW OF CONSERVATION of
ENERGY
Why won’t the beginning energy ever equal the
end energy???
Hint: Can heat be contained in an open system?
NO… please remember that energy is not
destroyed, but some of it can escape from the
system to the surroundings in the form of heat.
This is why no system will ever be 100%
efficient… meaning the beginning energy will
never equal the end energy…there is no such
thing as a frictionless system!!!
Efficiency
Let’s talk about reasonable efficiencies…
Can any system be 100% efficient?
NO… b/c some energy is given off from the
system in the form of heat as a product of
friction.
Would it be reasonable to have a system that has
a very low efficiency?
No… b/c then you are wasting energy.
Efficiency
Let’s talk about reasonable efficiencies…
Of the following efficiencies, which is most
reasonable?
.32%
100%
41%
89%
PE, KE, and TE Calculations
***For A-E calculate: PE, KE,
and TE (assume NO friction)
m = 2kg
g = 9.8 m/s2
Warm-Up
Explain in terms of forces and energy why a
ball does NOT bounce higher than the
point it was released from.
Power
Running does not require more work than
walking… so what is the difference between
running and walking?
A: The time it takes!!!
So we need to account for the time it takes to do
work… the equation that measures that is
Power: it equals the work done over time.
P=W
t
(Units for Power are J = Watts)
s
Power
1. While running track, Drew’s legs do 5780J of
work in 183s. What is his power output?
2. The chain that is pulling a rollercoaster up the
first hill does 24652J of work over a 79s time
interval. What is the power output of the chain?
3. It takes Ms. Webb 20s to apply 23N of force to
lift a box 5m. What was her power output?
Solving Energy Problems
Plug KE = ½ m * v2 into a Power Triangle
Write the equations when you solve for m and v.
Solving Energy Problems
½ m = KE
v2
v2 = KE
½m
KINETIC Energy Lab Data
Fill in on DATA TABLE 2
1. v = .140 m/s
2. v = 1.27 m/s
3. v = 1.74 m/s
4. v = .597 m/s
5. v = 1.74 m/s
WORK
Q: If you try to lift a car and it doesn’t
move…How much work have you done?
A: NONE!!! W = 0 J because the distance
was 0 m and W = F * (0 m) = 0 J